1. Field of the Invention
The present invention relates to a muffler of the type that integrally combines an exhaust manifold and a muffler (hereinafter, an exhaust manifold muffler) for an internal combustion engine.
2. Description of the Prior Art
Various types of exhaust manifold mufflers are proposed in, for example, Japanese Utility Model Publications SHO 60-43111 and SHO 56-85011 and Japanese Patent Publication 3-105011. More particularly, Publication SHO 60-43111 discloses a resonance type muffler structure integral with a catalytic converter installed below exhaust ports of an engine. Publication SHO 56-85011 discloses a muffler having an exhaust gas inlet connected to a first end of a perforated pipe, a first expansion chamber to which a second end of the perforated pipe opens, a second expansion chamber connected to the exhaust inlet via the perforated pipe, and an exhaust gas outlet connected to the second expansion chamber. Publication HEI 3-105011 discloses a muffler having a catalyst installed therein.
However, there are problems with the conventional exhaust manifold mufflers, such as increased exhaust gas flow resistance and amplified pressure pulsations between exhaust manifold pipes. For example, with the muffler of Publication SHO 60-43111, since the muffler is of a resonance type, the flow resistance is large. With the muffler of Publication SHO 56-85011, since the exhaust gas flows into the first expansion chamber, then reverses flow from the first expansion chamber to the perforated pipe and finally flows to the second expansion chamber, turbulences are generated within the perforated pipe that increase the flow resistance and hence the exhaust back pressure of the engine is increased. With the muffler of Publication HEI 3-105011, when the exhaust gas flows to the catalyst, the back pressure of the engine will be increased.